The present invention relates to an inverter apparatus and an uninterruptible power supply and more particularly to inverter apparatus and uninterruptible power supply suitable for stably operating inverters in parallel by suppressing an inverter output overcurrent during transition.
When driving a load of large capacity with power supply apparatus of small capacity or when constructing a highly reliable power supply system by multiplexing power supply apparatus in parallel, a method is adopted in which a plurality of power supply apparatus are operated or brought into running with their output terminals connected in parallel. A conventional inverter parallel running method will be described by taking an uninterruptible power supply for instance. The uninterruptible power supply is a power supply apparatus which, normally, receives power fed from a commercial alternating current system but in the event of power failure utilizes power of a storage battery to supply AC power of predetermined voltage and predetermined frequency to a load without causing interruption of service, and typically it comprises an uninterruptible power supply unit including a rectifier (converter), an inverter and a storage battery and a control circuit for controlling the uninterruptible power supply unit. The rectifier converts AC power fed from the commercial alternating current system into DC power which in turn is used to charge the storage battery and also as an input to the inverter. In the event of power failure, the storage battery substitutes for the rectifier in order to supply DC power to the inverter. Then, the inverter receives the supply of DC power from the rectifier or storage battery to deliver stable AC power of predetermined voltage and predetermined frequency. When connecting a plurality of uninterruptible power supply of the above construction in parallel and operating them, differences in voltage and phase between inverter output voltages of the uninterruptible power supply must be suppressed to minimize a current flowing between the uninterruptible power supply, that is, a cross current with the aim of protecting the power supply apparatus from overcurrent. Conventionally, a method for suppressing the cross current has been known as disclosed in JP-A-1-255475, according to which a phase difference and a voltage difference between output voltages of the individual inverters are detected, whereby the frequency of each inverter output voltage is so corrected as to suppress the phase difference and the magnitude of each inverter output voltage is so corrected as to suppress the voltage difference. There are available various kinds of methods for detection of phase difference and voltage difference, including a method disclosed in the aforementioned Laid-open Patent Application according to which a phase difference is detected indirectly by using a difference between effective power levels delivered out of the individual inverters and a voltage difference by using a difference between reactive power levels.
When the above method for detection of phase difference and voltage difference is adopted, the phase difference and voltage difference have to be detected in terms of DC components and therefore a filter having a time constant of several of tens of milliseconds is used in a detection circuit. This can ensure that a cross current flowing upon parallel throw-in of the inverters can be suppressed during a time interval of several of tens of milliseconds by the impedance of a main circuit on the output side of the inverter. But with recent improvements in performance of power semiconductor devices, there has arisen a tendency toward the use of a high frequency inverter in which the switching frequency of inverter is raised aiming at miniaturization of the filter adapted to decrease ripples in inverter output. In this type of high frequency inverter, the main circuit impedance on the inverter output side is low and hence it is difficult to suppress overcurrent on the inverter output side by using only the main circuit impedance. If an instantaneous voltage control scheme is adopted in which voltage of an output filter is instantaneously matched with a command value with a view of taking full advantage of high-speed response characteristics of high frequency inverter, a decrease in output voltage due to an overcurrent on the output side is corrected instantaneously and therefore suppression of an overcurrent due to impedance of the output filter cannot be expected.